2.0TDI high pitch noise under power

Hi all, 

My octavia tdi 4x4 has developed a strange noise. It sounds like the noise a worn out accessory belt would make under load (rubber screeching). My mechanic had a look and he thinks it seems to be coming from the rear of the engine and sounds more like high pressure air leak. 

The thing is, the noise happens when first taking off under load, then it goes away after 20 seconds or so and won't come back until turning the car off and then on again. 

The mechanic also seemed to think that there were two broken exhaust manifold studs and that as the car warms up, the leak might get sealed by the expanding metal. The thing is, the squeal/ whistle will go away after 30 seconds of driving when the temp needle has barely moved but if I turn the car off while hot, and back on, then the noise usually returns for the first few hard accelerations and then goes away again. With city (stop start) once the noise has gone, it usually doesn't come back as long as the car is accelerating and decelerating frequently, if I drive on the freeway for 30mins after the noise has gone, then when decelerating off the freeway and accelerating again, the noise will return momentarily. 

Anyone else ever had such issues or know what could be causing this. 

Its driving me nuts!